In precision polishing in processes of producing glass optical elements, glass substrates, and semiconductor devices, abrasive materials composed of oxides of rare earth elements, mainly composed of cerium oxide and additionally containing lanthanum oxide, neodymium oxide, praseodymium oxide, and/or oxides of other rare earth elements, have been traditionally used. Although other abrasive materials, for example, diamond, iron oxide, aluminum oxide, zirconium oxide, and colloidal silica are also known, cerium oxide has been widely used from the viewpoint of the high polishing rate and the surface flatness of polished workpieces. However, cerium oxide is unevenly distributed over the world and has not been stably supplied. Accordingly, there is a demand for establishing a method of producing an abrasive material that allows polishing with high accuracy in spite of a low cerium oxide content.
A typical method of producing a high-purity cerium oxide abrasive material for precision polishing in, for example, an optical glass-producing process, involves addition of a salt such as carbonate, oxalate, or acetate to an aqueous solution of, for example, purified cerous nitrate, cerous chloride, or cerous sulfate to precipitate the product such as cerous carbonate, cerous oxalate, or cerous acetate; collection of the precipitate by filtration; and drying then firing the precipitate to give cerium oxide.
For example, Non-Patent Literature 1 proposes a method of preparing particles having a narrow particle size distribution by heating an aqueous solution mixture of an aqueous solution of cerium nitrate, a solution of yttrium nitrate, and urea with stirring.
Patent Literature 1 describes an abrasive material including composite abrasive particles each having a core layer that is a base particle composed of an inorganic material having a specific gravity smaller than that of cerium oxide and a shell layer that is composed of microparticles, having a particle diameter smaller than that of the base particle and containing cerium oxide, bonded onto the surface of the base particle with a binder. According to the description, this abrasive material is produced by a method including a first preparation step of preparing a first mixture by adding aluminum oxide sol serving as a binder to a dispersion of silicon oxide particles as base particles with stirring; a second preparation step of preparing a second mixture by adding a dispersion of cerium oxide particles to the first mixture with stirring; a solid-liquid separation step of separating the solid composed of the base particles (silicon oxide) and the microparticles (cerium oxide) bonded to the base particles via the binder (aluminum oxide) formed in the second preparation step; a firing step of firing the separated solid at 700° C. to 900° C.; and a powdering step of pulverizing the resulting fired product with a dry jet mill. In this method, a polishing accuracy and a polishing rate equivalent to those of conventional products can be achieved, while the amount of cerium oxide to be used being reduced, by using particles each having a core-shell structure composed of a base particle (core layer) composed of silicon oxide and a shell layer containing cerium oxide bonded onto the surface of the base particle with a binder.